Roll forming multiple gauges with precision

Compensating for gauge changes in precision roll forming doesn't necessarily require significant investment. Spaces, the use of gauge spacers, and other technique can help when roll forming parts of multiple gauges.

Photo courtesy of Dietrich Metal Framing, www.dietrichindustries.com.

Sometimes a shape to be roll formed has multiple gauges. To reduce the cost of roll tooling and the changeover time, companies prefer to purchase just one set of tooling to do them all. But roll forming is a gauge-sensitive process, so one set of roll tooling might not deliver satisfactory results. So, the roll tooling design engineer needs to bridge the gap, accommodating multiple gauges while keeping costs under control. This is where gauge compensation can help.

Why Compensate?

When a shape has multiple gauges, it often keeps the outer dimensions, such as the width, height, and inner radii (see Figure 1). But to maintain these dimensions requires a slightly different strip width, as illustrated by the following formula, which, to keep things simple, assumes the neutral and central radii are identical (For more on these terms, see Radii in Roll Forming sidebar):

ΔS= -1.215 x N x ΔT, where ΔS is the change in strip width, N is the number of 90-degree bends in the shape, and ΔT is the gauge difference. (Formula 1)

The greater the gauge difference, the greater the change in strip width; likewise, the more bends in a shape, the greater the change in strip width. Note the negative sign in the formula. When the gauge decreases, ΔT becomes negative, or thinner, and the strip width (ΔS) becomes positive, or wider, and vice versa.

ADVERTISEMENT

Consider the C channel in Figure 1 and Figure 2. When the gauge changes from 0.082 inch to 0.030 in., ΔT = -0.052 in. There are four bends, so N=4. Thus, ΔT = +0.253 in. This means that the strip width will be 0.253 in. wider.

However, the neutral radius is always smaller than the central radius, a phenomenon called neutral radius inward moving. When the gauge is heavy, neutral radius inward moving is large (see Figure 1B); when the gauge is light, the move is small (see Figure 1C). As a result, the change in strip width becomes even greater, as shown by the following:

ΔS ≈ -1.607 x N x ΔT (Formula 2)

For the C channel shown in Figures 1 and 2, the final change in strip width is ΔS ≈ +0.333 in. The first pass often has traps in the roll that need to be adjusted when the strip width is changed. This is one situation where gauge compensation might be needed.

Bend locations also are gauge-sensitive. In the C channel example, there are two bends on each side of the forming centerline. The upper bend is formed first, then the lower bend. As shown in Figure 2, two strip thicknesses—0.082 and 0.030 in.—require different strip widths and slightly different bend locations to achieve the same roll formed outside dimensions.

Bend locations also are gauge-sensitive. In the C channel example, there are two bends on each side of the forming centerline. The upper bend is formed first, then the lower bend. As shown in Figure 2, two strip thicknesses—0.082 and 0.030 in.—require different strip widths and slightly different bend locations to achieve the same roll formed outside dimensions.

Formula 2 still can be used to estimate the amount of this change, but the N variable has a different meaning. In this case, it stands for the number of bends from the forming centerline to but not including the subject bend. So in the Figure 2 example, N=1, so ΔS ≈ -1.607 x 1 x (-0.052 )= + 0.085 in. So, the location of bend changes by +0.085 in. when the gauge changes from 0.082 to 0.030 in. This can cause error in the height of the C channel and, thus, must be compensated for in the tooling. Otherwise, the height will be 0.085 in. shorter for 0.030-in. gauge, bringing the part out of tolerance.

The rolls are positioned to accommodate the heaviest gauge. However, a typical roll former can adjust clearance by moving the rolls only up or down. The horizontal roll clearance stays set at the heaviest gauge. This means a lighter gauge in these rolls will exit the roll former with a lazy bend instead of a square bend that is equal on both legs (see Figure 3).

When to Compensate

It's easy to design for the heaviest gauge and let all other lighter gauges go through the same set of tooling. Some operations have run this way for years, but with accepted problems—issues that can be eliminated with gauge compensation. In such cases, the existing design can serve as a reference, but some changes might be needed.

A start-from-scratch job requires a detailed evaluation to uncover which errors occur and whether those errors cause the part to be formed out of tolerance. Although V angles don't require gauge compensation, most other cases aren't so clear-cut. In fact, parts with many bends, a wide range of gauges, or tight tolerances will all likely benefit from gauge compensation.

The evaluation should also consider the roll formed parts' use, which helps determine what tolerance on the part drawing must be satisfied and whether a lazy bend will conflict with another part in an assembly. When it is difficult to predict how much an error will affect the final formed part, it's usually best to be on the safe side and compensate.

Strategy of Gauge Compensation

Although it's ideal to compensate at every pass, this also increases downtime for changeovers. As a compromise, compensate at the critical passes only. The vase in Figure 4, for example, is compensated at the first seven passes for the three bends on top and pass 16 for the two bends on bottom, where they are overbent and finished.

Try to divide multiple gauges into groups, each with a narrow gauge range. This way, the operation enjoys the benefits of gauge compensation while avoiding the downtime required for making adjustments for each and every gauge change. The part in Figure 4, for instance, involves four gauges: 22 (0.029 in.), 20 (0.035 in.), 18 (0.045 in.), and 16 (0.056 in.). These can be divided into three groups: gauges 22 and 20; 18; and, finally, 16.

How to Compensate

The roll forming process can compensate for gauge variation through several means, some more expensive than others.

Gauge-specific Roll Pieces. Sometimes gauge-specific roll pieces are necessary, especially when an operation has more than one bend to perform or hold. The vase example uses such a setup, as shown in Figure 4C: Roll 6B2-#16 (sixth pass, second bottom roll, for 16-ga. material) can be switched out with the 6B2-#20 roll made for a 20-ga. strip.

Supermills. These roll form machines—which can move rolls along the horizontal shaft with a keystroke at the controller or a wrench at the roll station, depending on the machine—can be designed and built for a family of sections, such as those C channels. The rolls can be moved along the shaft to adjust for the location of bends as well as the strip width. The horizontal clearance can be adjusted automatically—on all the stands simultaneously— while the vertical clearance is set.

Gauge Spacers. Gauge compensation for many jobs, however, can be done with less expensive options that use existing rolls and machinery. This includes using gauge spacers that change the roll's position along the shaft, moving the roll over just enough to compensate for the new gauge. Gauge spacers come in one-piece sections designed for a specific gauge compensation, and can be listed on a gauge-specific setup chart. To further reduce downtime, horseshoe-shaped spacers allow operators to change out gauge spacers without removing all the roll tooling off the shaft.

Note that although standard shims also can be used for gauge compensation, they have significant drawbacks. They can be easily lost, and several shims placed together can produce inaccurate results.

One spacer can be used for one gauge group. In Figure 4C, the spacer 6TG1-16 adjusts the two top rolls—6T3 and 6T2—for 16-ga. material. When changing over to 20 ga., the operator simply replaces the existing spacer with one for the 20- and 22-ga. group (labeled "6TG1-20" in the figure).

Roll Flipping. For just two gauge groups, gauge compensation can occur by flipping the roll. The roll has the same corner radii on both sides, but the hub extends out just enough to compensate for the gauge change. In Figure 5, roll 2T2 has a hub extension of 0.137 in., while roll 2B2 has a hub extending out 0.085 in. In one position, the 0.137-in. hub extension lines up to work with the heavier gauge, as shown in Figure 5B. When flipped, as in Figure 5C, the 0.085-in. extension lines up to process the lighter-gauge strip.

Precision in Roll Forming

Such compensation solves several inherent problems in roll forming. First, roll tooling can be adjusted only up and down, not in and out. Second, gauge changes alter the neutral radius, which in turn changes the bend locations. If a part has few bends, a narrow gauge range, or wide tolerances, gauge compensation may not be necessary. But if the part has several bends, a wide gauge range, or tight tolerances, such compensation definitely can benefit an operation—and can save a lot of manufacturing headaches downstream. n

Radii in Roll Forming

Roll forming involves two types of radii, each representing a different point within the cross section of the material's thickness.

The central radius runs along the thickness midpoint, with half the material thickness on either side of the radius line.

The neutral radius represents the point in the material thickness where metal neither elongates nor compresses.

On the outer side of the neutral radius, the metal elongates, and on the inner side metal compresses. The neutral radius of all the bends will vary with any gauge change, as will the length of related straight segments.

Defining a Lazy Bend

A lazy bend has the following features:

A long vertical leg, which can be about 2 to 6 times as long as the horizontal leg

A bigger radius than a square bend

Springs back more not only because of the light gauge, but also the bigger radius; this can push the angle out of tolerance

More in Roll Forming from TheFabricator.com

Published In...

The Fabricator®

The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.